I have Arial Regular Italics installed in my Fonts folder in the OS Library. Font Book says it is ok.

In Scribus, it is not visible. I have even added a link to the Fonts folder in the Scribus Pref. Fonts section. Other Arial fonts are visible/available in Scribus. I'm running OS 10.12.6 and Scribus 1.4.6

Can someone help me figure this out or point me to a source where this is already covered?

I'm not sure what you mean by "Arial Regular Italics"."Arial Regular" is a font variant, and so is "Arial Italic" but "Arial Regular Italics" doesn't sound right to me.A font variant can be either "Regular" - usually upright - or "Italic" - slanted (basically) - but not both at the same time.

If you have a font installed on your machine and Scribus doesn't let you use it then it's usually because there's something wrong with the font. Scribus scans every installed font to make sure it can handle every possible situation. This is to make sure you don't get any weirdness along the way. Some fonts are just not created properly for typographical purposes and so Scribus will not let you use them. (This isn't normal for Arial so that's a bit strange.)

If Font Book says that the font is "okay" (whatever that means) then that's just OS X saying that it can use the font in general ways. However, general font use is not the same as the font being usable for proper typographic work.

If Scribus doesn't let you use a particular font - if that variant is not available from the drop-downs - then, mostly, you've just got to live with it and there's no way to force things otherwise.

You shouldn't need to link Scribus to the system Fonts folder. It should find that folder itself automatically.

Try and find out if you have more that one set of Arial fonts installed. Sometimes, especially after an OS upgrade, you can have different versions of the same font installed and Scribus might be using the "wrong" version.

If that doesn't work - or it isn't the case - then you might have to supply some screenshots showing people more information about your situation.